Physical mechanisms causing the efficiency droop in InGaN/GaN blue light-emitting diodes and remedies proposed for droop mitigation are classified and reviewed. Droop mechanisms taken into consideration are Auger recombination, reduced active volume effects, carrier delocalization, and carrier leakage. The latter can in turn be promoted by polarization charges, inefficient hole injection, asymmetry between electron and hole densities and transport properties, lateral current crowding, quantum-well overfly by ballistic electrons, defect-related tunneling, and saturation of radiative recombination. Reviewed droop remedies include increasing the thickness or number of the quantum wells, improving the lateral current uniformity, engineering the quantum barriers (including multi-layer and graded quantum barriers), using insertion or injection layers, engineering the electron-blocking layer (EBL) (including InAlN, graded, polarization-doped, and superlattice EBL), exploiting reversed polarization (by either inverted epitaxy or N-polar growth), and growing along semi- or non-polar orientations. Numerical device simulations of a reference device are used through the paper as a proof of concept for selected mechanisms and remedies.

Efficiency droop in InGaN/GaN blue light-emitting diodes: Physical mechanisms and remedies

The paper reviews applications of ABC-model to interpret internal quantum efficiency and its droop in III-nitride light-emitting diodes. Advantages of the model, its intrinsic limitations, and tentative mechanisms responsible for deviation of the model predictions from available observations are discussed. New experimental information on recombination processes in the LED active regions coming in terms of the ABC-model is considered along with still open questions and tasks for further experimental and theoretical studies.

ABC-model for interpretation of internal quantum efficiency and its droop in III-nitride LEDs: a review

High-power flip-chip light-emitting diodes (FCLEDs) suffer from low efficiencies because of poor p-type reflective ohmic contact and severe current crowding. Here, we show that it is possible to improve both the light extraction efficiency (LEE) and current spreading of an FCLED by incorporating a highly reflective metallic reflector made from silver (Ag). The reflector, which consists of an Ag film covered by three pairs of TiW/Pt multilayers, demonstrates high reflectance of 95.0% at 460 nm at arbitrary angles of incidence. Our numerical simulation and experimental results reveal that the FCLED with Ag-based reflector exhibits higher LEE and better current spreading than the FCLED with indium-tin oxide (ITO)/distributed Bragg reflector (DBR). As a result, the external quantum efficiency (EQE) of FCLED with Ag-based reflector was 6.0% higher than that of FCLED with ITO/DBR at 750 mA injection current. Our work also suggests that the EQE of FCLED with the Ag-based reflector could be further enhanced 5.2% by replacing the finger-like n-electrodes with three-dimensional (3D) vias n-electrodes, which spread the injection current uniformly over the entire light-emitting active region. This study paves the way towards higher-performance LED technology.

Highly efficient GaN-based high-power flip-chip light-emitting diodes

We discuss the challenges of light-emitting diodes in view of their application to solid-state lighting. The requirement is to at least displace the quite efficient fluorescent, sodium, and high intensity discharge lamps used today in the main energy consuming lighting sectors, industrial, commercial and outdoors, with more efficient and better light quality lamps. We show that both from the point of view of cost of ownership and carbon emissions reduction, the relevant metric is efficiency, more than the cost of lumens. Then, progress from present performance requires identification of the loss mechanisms in light emission from LEDs, and solutions competing with mainstream c-plane LEDS grown on sapphire need to be on par with these. Special attention is devoted to a discussion of the efficiency droop mechanisms, and of a recent direct measurement of Auger generated electrons which appear to be responsible for droop.

The efficiency challenge of nitride light-emitting diodes for lighting

From Large-Size to Micro-LEDs: Scaling Trends Revealed by Modeling

In this paper, we report on the results of experimental and theoretical study of a promising way for suppression of the efficiency droop with current in InGaN-based light emitting diodes. Simulations carried out using a drift-diffusion approach with quantum-mechanical corrections clearly show that non-radiative Auger recombination is the principal mechanism limiting the device performance at high-injection level. New design of LED heterostructure with short-period superlattice in the active region is proposed and assessed theoretically. Experimentally, the implementation of the structure design in high-power devices has resulted in substantial suppression of the efficiency droop compared to conventional multiquantum-well InGaN LEDs.

Efficiency droop suppression in InGaN‐based blue LEDs: Experiment and numerical modelling

The paper discusses various factors affecting internal quantum efficiency (IQE) of state-of-the-art III-nitride lightemitting
diodes (LEDs). A general figure of merit for LED heterostructures, namely the quality factor, is proposed on the
basis of a simple recombination model, which enables comparison of overall performances of the structures either
emitting light at different wavelengths or having substantially different designs. The relationships between the quality
factor, maximum value of IQE, and IQE droop with the current density are revealed. Some ways for IQE improvement
and reducing its droop are considered. Among them, the use of short-period superlattice (SPSL) active regions is found
to be quite promising. The operation of such structures and their properties are examined in detail by simulations
accounting for quantum corrections to electron and hole transport within the quantum-potential concept.

Simulation of light-emitting diodes for new physics understanding and device design

Using coupled 3D modelling of electrical, thermal, and optical processes, we consider operation of a blue LED with advanced thin-film flip-chip design. Significant effect of current crowing on light extraction from the LED die is predicted theoretically. As a result, the light extraction efficiency is found to decrease remarkably with the operation current. The nature of the effect is identified, and some ways for improvement of the high-current extraction efficiency are suggested and examined by simulations. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

/pdf/current-crowding-effect-on-light-extraction-efficiency-of-2br9u1bdwi.pdf

Current crowding effect on light extraction efficiency of thin‐film LEDs

Operation of blue light emitting diodes with ITO spreading layers of various thicknesses deposited on top of the p-contact layer is considered by simulations. Effects of the ITO layer on the current spreading in and the efficiency of light extraction from the LED dice are examined in detail. The electrical LED parameters, operation voltage and series resistance at a certain forward current, are found to be improved with the thickness of ITO films. In contrast, the optical parameters, external quantum efficiency and output power, become reduced with the thickness because of free-carrier light absorption in the ITO films. These opposite trends make practically unrealistic simultaneous optimization of the electrical and optical parameters by varying the ITO layer properties only. An alternative way is suggested for improvement of the LED performance, based on optimization of light extraction from the LED due to a proper choice of the ITO parameters and of current spreading in the die due to an increase of doping level in the n-contact layer. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

/pdf/effect-of-ito-spreading-layer-on-performance-of-blue-light-19y6w5qmm9.pdf

O. V. Khokhlev

Papers

Efficiency droop suppression in InGaN‐based blue LEDs: Experiment and numerical modelling

Simulation of light-emitting diodes for new physics understanding and device design

Current crowding effect on light extraction efficiency of thin‐film LEDs

Effect of ITO spreading layer on performance of blue light‐emitting diodes